1. Field of the Invention
The invention relates in general to the structure of monitoring tool for photolithography and etching process. More particularly, this invention relates to the structure of a critical dimension bar (CD bar).
2. Description of the Related Art
In each stage of semiconductor fabrication process, the photolithography and etching process is one of the most crucial steps that determine the product performance. The objective and for photolithography process is to transfer the pattern of a photoresist layer to a material layer. The process comprises forming and patterning the photoresist layer on the material and performing an etching process on the material layer using the patterned photoresist layer as a mask. As a result, the material layer is patterned. When a width of the pattern of the material becomes an important parameter of the electronic device (or when the width of such pattern is the narrowest among all the patterned layers on the wafer), the width is called critical dimension. The material layer having the pattern with such width is also referred as a critical material layer. Since the critical dimension plays an important role for the product characteristics, the error of the critical dimension has to be controlled within a certain range to avoid the deterioration of devices.
Referring to FIG. 1, in a conventional photolithography and etching process, to monitor the error of critical dimension for patterned critical material layer, critical dimension bars 120 with simple pattern of the critical material layer are formed on the scribe lines. The scribe lines are the dot-point lines between the dies 110 on the wafer. This is because the pattern of the integrated circuits on the surfaces of the dies 110 are normally very complex, it is thus very difficult to directly measure the critical dimension of the critical material layer. Therefore, one has to measure the critical dimension bars 120 to determine the critical dimension of the critical material layer on the dies 110.
FIG. 2A shows a schematic drawing of a cross section view of a conventional critical dimension bar. The fabrication process is briefly described as follows. In the photolithography and etching processes for various layers before forming the critical material layer, the applied photomasks do not comprise the pattern of the critical dimension bar 120. Therefore, the surface of the substrate 200 for forming such critical dimension bar 120 is flat. A critical material layer 210 is formed on substrate 210 on which the critical dimension bar is to be formed while such critical material layer 210 is formed on the dies. A photolithography and etching process is performed to form a patterned photoresist layer (not shown) on the critical material layer 210. A photomask with an opening pattern similar to that over the die is used to expose the photoresist layer as an example. The pattern width of both photomasks to expose the critical material layer over the die and the critical material layer 210 for patterning the critical dimension bar 120 is the same, however, the pattern for forming the critical dimension bar is simpler.
After exposure, development, baking and other subsequent processes are performed to form the openings 220a in the critical material layer 210. As the critical dimension bar 120 is formed with a pattern width as that of the critical material over the die, the critical dimension of the openings 220a formed on the die 110 as shown in FIG. 2B can be obtained.
Referring to FIG. 3, in a conventional fabrication process, several patterning processes have been performed in the die area 110 on the substrate 300 prior to formation of the critical material layer 210. Therefore, an uneven surface is resulted as shown. When a photoresist layer is coated and exposed on the critical material layer 210, a focus offset between the elevated portion (the P area) and the lower portion (the Q area) is resulted. That is, the distances between the focal point of the exposure light source and the bottom surface of the photoresist layer 215 at the P area and the Q area are different from each other. Consequently, referring to FIG. 3B, the opening 220b formed on the P area has a width d1 different from the width d2 of the opening 220c formed on the Q area. That is, the critical dimensions of the openings 220b and 220c are different from each other.
As mentioned above, since the critical dimension bar 120 is formed on a substrate surface directly. Without the uneven surface as the die region on the substrate, the exact critical dimension can hardly be precisely simulated via the critical dimension bar.
In addition, referring to FIG. 2B, several openings 220a are distant away from each other. In the conventional critical dimension bar, only the openings 220b/c close to each other can be formed. It is inevitably to cause deviation of the critical dimensions of these openings since the exposure light beams traveling through the neighboring opening patterns of the photomask may interfere with each other.
The invention provides a structure of a critical dimension bar to solve the problems occurring while using the conventional critical dimension bar. The critical dimension bar comprises an uneven surface and non-uniformly distributed patterns similar to those of the die, so that the critical dimension of the die can be precisely determined. The structure is formed on a substrate between neighboring dies. The structure comprises a base layer and a patterned critical material layer. The base layer is formed on the substrate to result in a same surface profile as the die. The critical material layer on the die, the base layer and the substrate surface comprises the die pattern, a first test pattern and a second test pattern transferred from the die photomask pattern, the first test photomask pattern and the second test photomask pattern, respectively. These patterns comprise similar type of patterns and the same width.
The invention further provides a method for fabricating the critical dimension bar. A base layer is formed on a portion of the substrate, such that the portion of the substrate has the same surface profile as the die. A critical material layer is formed on the die and the portion of the substrate. Using a photomask comprising a die photomask pattern, a first test photomask pattern and a second test photomask pattern to perform an exposure step, a die pattern, a first test pattern and a second test pattern are formed in the die and the portion of the substrate. The die photomask pattern, the first test photomask pattern and the second test photomask pattern all comprise a same type of pattern with same width.
In addition, in the above structure and method provided by the invention, when the die pattern comprises a first region and a second region with different density of patterns distribution, the first and second test pattern also comprises similar structures as these two regions. As a result, the pattern distribution on the die can be precisely simulated to monitor the influence of the variation in critical dimensions.
The above critical material layer comprises an insulation layer, and the die pattern comprises a contact window pattern or a via hole patter. The critical layer may also comprises a conductive layer such as a polysilicon layer or other conductive material, and the die pattern may comprises a gate line pattern or a conductive line pattern.
The invention further provides a method to monitor the critical dimension deviation. A critical dimension bar with a first test pattern and a second test pattern is provided. The first test pattern and the second test pattern are compared to each other to obtain a deviation of critical dimension of the die pattern. If the die pattern comprises a first region and a second region with two different density of pattern distribution, the first test pattern and the second test pattern corresponds to the patterns of the first and second regions, respectively. The deviation in critical dimension for the first and the second regions of the die pattern can thus be inferred.
As mentioned above, the structure and method for forming the critical dimension bar provided by the invention comprises at least the following the advantages. The design of the critical dimension bar comprises the same surface profile as the die, that is, the same surface height difference as the die, so that the actual pattern of the die can be precisely simulated. The deviation of critical dimension caused by the surface height difference can thus be monitored. In addition, the base layer comprises a first and a second test pattern to correspond to the die pattern with different density of distribution. The variation in critical dimension can thus be precisely reflected.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.